The SOS repair mechanism enables bacterial cells to respond to overwhelming DNA damage, particularly from UV light, by activating a systematic repair response involving over 30 genes. Key proteins in this process include RecA and LexA, which regulate the expression of genes involved in DNA repair, such as UvrA, UvrB, and UmuC. The mechanism ensures that cellular repair occurs prior to cell division, although it can be error-prone due to the activation of bypass mechanisms.

1. SOS REPAIR MECHANISM
Submitted by : Varnit Chauhan
Integrated B.Tech+M.tech
5th sem.

2. SOS REPAIR
 Stand for ‘Save Our Ship’.
 Occurs when cells are overwhelmed by UV
damage.
 Triggered when other repair systems are
overwhelmed by amount of damage.
 The error-prone(SOS) repair mechanism involves
DNA pol. III and 2 other gene products encoded by
umuCD.

4. THE SOS RESPONSE
 In response to extensive genetic damage there is a
regulatory system that co-ordinates the bacterial cell
response. This results in the increased expression of
>30 genes, involved in DNA repair, these include:
 recA - activator of SOS response, recombination
 sfiA (sulA) - a cell division inhibitor (repair before
replication)
 umuC, D - an error prone bypass of thymine dimers
(loss of fidelity in DNA replication)
 uvrA,B,C,D - excision repair
 The SOS response is regulated by two key genes:
recA & lexA

6. SOS RESPONSE IS CONTROLLED BY LEXA AND
RECA
recA lexA target gene
LexA
RecA
LexA
LexA
e.g.recA, lexA, uvrA, uvrB, umuC
Repressed
SOS response is controlled by LexA and RecA
OFF
ON
recA lexA target gene
LexA
e.g. RecA, UvrA, UvrB, UmuC
de-repressed
RecA is activated in the presence of damaged DNA. It serves as a co-protease to activate a latent,
self-cleaving proteolytic activity in LexA, thereby removing the repressor from SOS inducible genes.
RecARecA RecA
RecA
+ cleaved LexA
active proteins